The Third Generation Partnership Project (3GPP) New Radio (NR) system (Fifth Generation (5G)) employs mmWave transmissions. The carrier frequency will be in the range of 24 GHz to 100 GHz. At such high frequencies beam forming is applied at a base station (called “gNB”) and a user equipment (UE) to compensate for the high path loss. At the UE, receiver (RX) beam forming may be implemented via analog beam forming. Digital beam forming may not be feasible at the UE for the high number of receive antennas because they share a single analog-to-digital converter (ADC). A typical RX beamforming implementation may be done via phase shifters per receive antenna element and summation of the analog phase shifted signals. The combined signal is converted to digital by an ADC via sampling.
Given a non-varying transmit side beamforming at the gNB, the UE needs to optimize the analog beamforming phase shifts and switches to use an antenna element or not to maximize the overall path gain. The 3GPP NR supports two reference signals for the RX beam management at the UE: a synchronization signal block (SSB) and a channel state information reference signal (CSI-RS).
Previous proposed solutions rely on a pre-optimized codebook for the phase shift values. The codebook is optimized in such a way to cover all possible directions of one antenna panel. A UE iteratively applies RX beam switching for pre-optimized codeword candidates and then measures the corresponding RX beam, until a best pre-optimized codeword is found. The codebook may consist of a small number of coarse beams and many sharp beams. The RX beam may be optimized by first searching for the best coarse beam and then searching for the best sharp beam within the sharp beams that are contained in the coarse beam. An alternative is to do a full search over all possible sharp beams.
Even though potentially optimized for a given hardware configuration and scenario, a codebook may not be optimal due to dynamically changing environmental influences and production tolerances for the antennas in the final form factor. Therefore, the correspondence between the coarse beams and the sharp beams cannot be always ensured. In addition, the optimum receive beam may no longer be a part of the pre-configured codebook at all. To guarantee a near-optimum beam in the pre-configured codebook, a sufficiently fine granularity of beam directions needs to be supported and hence the number of codebook entries that need to be tested may become substantial. This may result in an unacceptably long time to acquire a near-optimum beam.